The present invention relates to global positioning system (GPS) receivers, particularly portable GPS receiver units usable in connection with portable general purpose computers.
In the early 1970's, the U.S. government began development of a new satellite navigation system which has come to be known as the Global Positioning System (GPS). Although GPS is still undergoing experimentation and testing, GPS has been in practical use by the U.S. military and other specialized organizations for about 5 years and is expected to be declared fully operational within the next year or two.
As its name implies, GPS is a system which allows the user to precisely determine his or her location anywhere on earth. GPS is based upon satellite ranging. To accomplish this, a special GPS receiver is used to receive signals transmitted by a group of the orbiting satellites and thereby measure the distance between the receiver and each of the satellites within the group. The satellites act as precise reference points. To determine the user's position (latitude, longitude and altitude) the GPS receiver measures the distance to four satellites.
Each GPS satellite transmits by radio waves three primary signals, two of which correspond respectively to the current time and the satellite's position. The satellites determine their own position and the current time from on-board celestial navigation equipment and atomic clocks accurate to one second in 300,000 years. The third signal is a very long bit stream known as a pseudo-random noise code (PRN). The noise code is used by the receiver to calculate the range and position of three or four satellites. Once this is done, the GPS receiver can compute its own location by triangulation.
GPS receivers receive the satellite signals via a GPS antenna which must have a clear view of the sky. GPS antennas come in a variety of configurations, but typically comprise a coiled wire built into a relatively flat, e.g. saucer shaped, housing. Typically, the antenna is remotely located from the receiver unit and is connected thereto by a cable.
GPS has a myriad of present and potential future applications. Present applications include vehicle (e.g. ship, airplane and land vehicle) navigation and tracking, and surveying. GPS is also presently being used to disseminate precise time, time interval and frequency information (from the atomic clocks on board the satellites) to control timing signals and oscillators, e.g., in the communications and electric power industries.
GPS is being integrated with electronic mapping and charting systems as one of the latest steps in the evolution of navigational tools. Portable GPS receivers are being used in conjunction with portable computers to create detailed electronic facsimiles of street maps, for example, by tracking and recording a vehicle's movements. Once created, GPS and a microprocessor are used to display a vehicle's position against the background of the electronic map or chart.
The Global Positioning System has spawned a new industry for the production, sale and use of GPS receivers. Most of the receivers that have been offered are built into special purpose devices for navigation, surveying or other applications. Recently, GPS receivers units usable in connection with general purpose portable computers have been offered. Rockwell International has advertised an IC board called NavCore V (see GPS World, February 1992, page 13) which is adaptable for insertion into an expansion slot of a personal computer. Similar GPS expansion cards have been developed and offered by the following companies: Navstar Electronics, Magnavox and Koden Electronics. The marriage of GPS and portable general purpose computers allows for a flexibility in the application of GPS unattainable with the special purpose devices.
General Engineering & Systems S.A. (GESSA) has advertised a product called GPSpac, which represents an integration of the HP 95LX palmtop PC from Hewlett Packard and Rockwell's NavCore V five channel GPS receiver. See GPS World, January 1992, page 42. In the advertisement, the unit appears to be mounted underneath a palmtop computer with an antenna mounted off to the side. A problem with the GPSpac receiver unit is that it is bulky and not easily portable, due largely to the provision of a non-retractable GPS antenna connected to the outside of the main GPS receiver housing.